Gas turbine engines include a compressor section for supplying a flow of compressed combustion air, a combustor section for burning a fuel in the compressed combustion air to produce thermal energy, and a turbine section for converting the thermal energy into mechanical energy in the form of a shaft rotation. Many parts of the combustor section and turbine section are exposed directly to hot combustion gasses. For example, the combustor, the transition duct between the combustor and the turbine section, and the turbine stationary vanes, discs and rotating blades are all exposed to hot combustion gases.
It is also known from basic thermodynamics that increasing the firing temperature of the combustion gas may increase the power and efficiency of a combustion turbine. Modern high efficiency combustion turbines have firing temperatures that may be well in excess of the safe operating temperature of the structural materials used to fabricate the hot gas flow path components. Special superalloy materials have been developed for use in such high temperature environments.
These materials have been used with specific cooling arrangements, including film cooling, backside cooling and insulation. Superalloys are well known in the art of power generation. Superalloys are based on Group VIIIB elements and usually consist of various combinations of Fe, Ni, Co, and Cr, as well as lesser amounts of W, Mo, Ta, Nb, Ti, and Al. The three major classes of superalloys are nickel-based, iron-based, and cobalt-based alloys. Nickel-based superalloys can be either solid solution or precipitation strengthened. Solid solution strengthened alloys are used in applications requiring only modest strength.
A precipitation-strengthened alloy is required in the most demanding applications such as the hot combustion gas flow path sections of gas turbine engines. The primary strengthening phase in nickel-based superalloys is Ni3(Al, Ti), which is referred to as gamma prime. A characteristic of the gamma prime strengthened nickel-based superalloys is that they retain their strength at elevated temperatures and may be used in load-bearing structures to the highest homologous temperature of any common alloy system, being up to Tm=0.9, or 90% of their melting point.
Although nickel-based alloys are commonly used as the material for aircraft engine discs, application of these materials in industrial gas turbines results in significant technical problems as well as increased costs. For example, the manufacture of large industrial gas turbine discs via the conventional cast/wrought processing route is difficult. The starting ingots are already at their size limit for the avoidance of unacceptable segregation related defects, thus limiting the choice of available alloys. The number of presses with sufficient capacity to forge such discs is also extremely small. An alternative to cast/wrought processing is the powder metallurgy approach. Although powder metallurgy eliminates the need for casting large ingots, a high capacity forging press is still required. Neither the cast/wrought or powder metallurgy processing routes avoid the expense associated with the additional alloy cost.